1. Field of the Invention
The present invention relates to an implantable cardiac stimulating device and specifically a biventricular pacemaker capable of monitoring capture via implanted ventricular electrodes. The invention is also applicable for an Implantable Cardioverter Defibrillator, ICD, comprising a biventricular pacemaker.
2. Description of the Prior Art
Most pacers stimulate the right ventricle of the heart but it is also known to stimulate the left ventricle. In particular, for treatment of congestive heart failure combined with ventricular dyssynchrony, it is known to stimulate both ventricles, in order to optimize the hemodynamic performance of the heart.
The improvement in hemodynamic performance that can be obtained is due to improved synchronization between the right and left ventricles. If capture is lost on any of the two ventricular stimulating electrodes the beneficial effect of the biventricular pacing therapy is lost.
U.S. Pat. No. 5,740,811 discloses a device and method to synthesize an ECG that in appearance can be substituted for a surface ECG. The synthesized ECG is based on endocardial signals obtained from pacemaker heart electrode leads.
United States Patent Application Publication 2001/0049542 discloses a biventricular pacemaker in which a bi-chamber composite cardiac signal is analyzed to determine if biventricular capture is present.
U.S. Pat. No. 6,148,234 discloses a biventricular pacing system adapted to determine capture or not in both ventricular chambers. Loss of capture in one chamber is determined if an R-wave is detected in that particular chamber during a refractory period after the stimulation pulses.
United States Patent Application 2001/0049543 discloses a biventricular pacemaker utilizing a cross-chamber stimulation method. Biventricular capture can be determined in a cross chamber sensing configuration such as with a ring-to-ring electrode configuration.
PCT Application WO 01/74441 discloses an apparatus and method for verifying capture by a selected electrode in a multi-site pacemaker. A switching circuit switches the input of an evoked response channel to an unselected electrode. The presence or absence of capture is determined from the output of the evoked response sensing channel.
PCT Application WO 99/29368 discloses an implantable device that automatically verifies capture. During predetermined periods, the device utilizes two or more pacing/sensing electrodes positioned within an electrically continuous area of the heart, with one electrode being used to provide a pacing stimulus and the other electrodes being used to determine capture.
An Abstract in Pace “ECG characteristics of simultaneous Biventricular Stimulation in Canines” by Rick McVenes and Melissa Christie discloses that biventricular capture gives shorter QRS and QT durations and that the paced QRS axis is affected depending on whether there is single site or dual site biventricular capture. The abstract was published in PACE, Vol. 21, April 1998, Part II, page 893.
A problem with known biventricular pacemakers equipped with features to assure capture after stimulation pulses is that it is difficult to verify capture on every beat particularly if the pacemaker can operate with a delay between the stimulation of the first ventricle and the second ventricle. The reason is that the stimulation pulse to the second ventricle may make it impossible to detect capture from the stimulation pulse delivered to the first ventricle. After a delivered stimulation pulse the evoked response detection window typically ends 50–100 ms after the stimulation and a second stimulation pulse delivered during this period will make it difficult to detect the evoked response from the first stimulation pulse.